1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer product that enable high-accuracy calibration.
2. Description of the Related Art
When performing calibration of single colors by outputting single-color patches, color measurement is done by outputting the gradation patches of four colors of cyan (C), magenta (M), yellow (Y), and black (K). That results in an increase in the number of patches thereby leading to the following issues.
Usually, depending on image quality modes such as a high image quality mode or a standard mode, or depending on image attributes such as characters or photographs; a multifunction peripheral (MFP) implements different dither patterns. In an MFP, for each dither pattern, single-color gradation patches are output and scanned by a scanner for the purpose of calibration, or manual color measurement is done using an accompanying colorimeter for the purpose of calibration. However, if a large number of patch images are output in an MFP, then it is undesirable from the environmental perspective or from the economical perspective.
For the scanner to perform reading, the patch images that have been output on paper sheets need to be set one by one in the scanner, which requires time and efforts. Furthermore, when there are a large number of patches, a calibration system requiring manual color measurement using a colorimeter puts a lot of burden on the user.
Therefore, a technology is available in which outputting the single-color patches is replaced by outputting the mixed-color patches, and the color measurement values of the mixed-color patches are referred to for calculating the amount of correction required to ensure that the color values of the single colors correspond with target color values.
For example, Japanese Patent Application Laid-open No. 2003-094732 discloses that, when performing correction of an image recording apparatus, patches (test charts) that have a gray hue formed by a combination of substantially equal share of color pigments of cyan, magenta, and yellow, but have mutually different gray densities are printed as output. In Japanese Patent Application Laid-open No. 2003-094732, the patches are subjected to density measurement so as to obtain an integral density measurement value for each color pigment. Then, for each color pigment, an analytical density corresponding value is calculated from the difference between the integral density measurement value and an integral density reference value (target value). Moreover, the analytical density corresponding values are used to calculate conversion conditions (one-dimensional look-up table (LUT)) for regulating input signals for correction. Hence, in Japanese Patent Application Laid-open No. 2003-094732, it is possible to reduce the amount of recording material used in outputting test chart images.
However, in an apparatus in which mixed-color patches are output and the measurement values of the mixed-color patches are referred to for calculating the amount of correction required to ensure that the color values of the single colors correspond with target color values, sufficient attention is not given to performing total amount control of the mixed-color patches. That is, when it is assumed that there is 100% output of solid single colors, the total amount control in the electrophotographic technique is generally around 250%. Meanwhile, in the ink-jet technology, there are apparatuses with the total amount control around 150%. In the gray color obtained by superposing three colors, the total amount control is 300% at a maximum. Thus, in the abovementioned conventional technology, the calibration loses its accuracy in high-density areas on the high-density side because of the use of such mixed-color patch patterns that fall outside the total amount control range thereby not guaranteeing the output of stable colors.